Differential Steering to steer a tank

Why is it when a tread on one side of a tank stops the other side which is still moving will 'skid' in the direction of that tread? This is used to steer a tank but what is the physics behind this please

Since one side is being propelled forwards but the other is being held back by the ground, there's a torque and the tank will tend to turn towards the halted side.
Which point will be the centre of rotation depends on the distribution of weight. With an even distribution, I would expect it be somewhere within the halted track, but I'm not sure where.

I'm guessing that voyager221 means that a tank tends to skid "outwards" while turning with the inner tread stopped. I don't know if this is true. If so, it could be related to the fact that the tread is a flat track like surface instead of a round wheel. It could be due to the fact that the inner tread is sliding as well as the outer tread. I don't know if the surface, such a pavement versus sand, makes a difference in the outcome.

If one track is not powered then I can't think of a situation where the tank would do other than steer in the direction of the stationary track. I know that the opposite can happen with a motor car when one wheel actually stops rotating (aquaplaning) but, with a tank??? Not likely to get a situation like that.

If one track is not powered then I can't think of a situation where the tank would do other than steer in the direction of the stationary track.

The track is powered, but forced to not move. My guess is that the original post implies that the tank does not pivot about the stopped track, but turns and moves in a circle with a slightly greater radius, which would mean the inner track gets dragged forward a bit as the tank turns.

Do you mean 'powered' or 'braked'? But even if one track is powered (driven) a bit less than the other, you will still get some steering effect.
This method of steerind is very messy and destroys the road because there is a lot of scuffing.

My guess is that the original post implies that the tank does not pivot about the stopped track, but turns and moves in a circle with a slightly greater radius, which would mean the inner track gets dragged forward a bit as the tank turns.

The original post says "This is used to steer a tank", which suggests to me there's no subtle, unexpected effect being discussed.
From a quick web search, looks like, in the context of tracked vehicles, "steering", "skid steering" and "differential steering" are synonyms.
http://www.beam-wiki.org/wiki/Steering_Techniques [Broken] uses "differential" for the independent wheel case (as a car) and "skid" for tracked vehicles.

That site also claims that with one track stopped the turning centre will be at the centre of the stopped track's footprint. That puzzles me. Linear forces from the dynamic friction on the stopped track would all cancel, leaving only the forward force supplied by the moving track. So I would have thought the turning centre would be displaced somewhat away from the centre of the vehicle.

The track system is very inefficient, I think. I bet they don't freewheel very well.

They don't. Yet, you still have to apply clutch and brake to make the turns happen. I'm a bit fuzzy on details myself, since I've never operated one. Just what I've been told by my grandfather.

I think that when they want to turn really tightly, they will reverse one track

Only from being stopped. Just like a car going forward can't be put in reverse, you can't put a track in reverse gear while that track still rolls forward. You have to bring it to the stop, or at least near enough, and then reverse. At full speed, turns are done with brakes.

The simplest way i imagine it, imagine that the stopped track becomes an anchoring point, and then imagine the moving track is a moving object, now put a pin on a board, attach a string, and imagine the stopped track is the pin, the string is the actual tank, and the moving track is a pen attached to the string and the pin. As you move the pen, you can see you only have two options to move with the pen.